title: Rationalization of the barrier height for p-Z-styrene epoxidation by iron(IV)-oxo porphyrin cation radicals with variable axial ligands.
abstract: A versatile class of heme monoxygenases involved in many vital functions for human health are the cytochromes P450, which react via a high-valent iron(IV) oxo heme cation radical species called Compound I. One of the key reactions catalyzed by these enzymes is C═C epoxidation of substrates. We report here a systematic study into the intrinsic chemical properties of substrate and oxidant that affect reactivity patterns. To this end, we investigated the effect of styrene and para-substituted styrene epoxidation by Compound I models with either an anionic (chloride) or neutral (acetonitrile) axial ligand. We show, for the first time, that the activation enthalpy of the reaction is determined by the ionization potential of the substrate, the electron affinity of the oxidant, and the strength of the newly formed C-O bond (approximated by the bond dissociation energy, BDE(OH)). We have set up a new valence bond model that enables us to generalize substrate epoxidation reactions by iron(IV)-oxo porphyrin cation-radical oxidants and make predictions of rate constants and reactivities. We show here that electron-withdrawing substituents lead to early transition states, whereas electron-donating groups on the olefin substrate give late transition states. This affects the barrier heights in such a way that electron-withdrawing substituents correlate the barrier height with BDE(OH), while the electron affinity of the oxidant is proportional to the barrier height for substrates with electron-donating substituents.
authors.affiliation: Department of Applied Physics, School of Physical Sciences, Babasaheb Bhimrao Ambedkar University, Vidya Vihar, Rae Bareilly Road, Lucknow 226-025, India. dkclcre@yahoo.com
authors.forename: Devesh
authors.initials: D
authors.lastname: Kumar
authors.forename: Reza
authors.initials: R
authors.lastname: Latifi
authors.forename: Suresh
authors.initials: S
authors.lastname: Kumar
authors.forename: Elena V
authors.initials: EV
authors.lastname: Rybak-Akimova
authors.forename: Mala A
authors.initials: MA
authors.lastname: Sainna
authors.forename: Sam P
authors.initials: SP
authors.lastname: de Visser
mesh_terms: D002412:Cations / Q000737:chemistry; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry; D004852:Epoxy Compounds / Q000737:chemistry*; D006801:Humans; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D016877:Oxidants / Q000737:chemistry; D011166:Porphyrins / Q000737:chemistry*; D020058:Styrene / Q000737:chemistry*
chemical_list: D002412:Cations; D004852:Epoxy Compounds; D008024:Ligands; D016877:Oxidants; D011166:Porphyrins; C066317:ferryl iron; D020058:Styrene; D003577:Cytochrome P-450 Enzyme System; D007501:Iron